1. Field of the Invention
The present invention relates generally to RF devices, and particularly to impedance transforming network.
2. Technical Background
Impedance matching is an important aspect in the design of microwave and millimeter wave circuits. A good impedance match ensures an efficient transfer of power from the source to the load. Conversely, a mismatch between the load and source results in reflections that degrade the system signal to noise ratio (SNR) and causes the sensitivity of the device to deteriorate. The reflections generate a standing wave along the transmission line. Standing waves are problematic in high power applications because they lead to relatively high currents at certain spots along the transmission line. As those skilled in the art will appreciate, the current is dissipated as heat in accordance with the relationship I2 R, where I is the current and R is the resistance of the transmission line. The extraordinary heat created at these so-called “hot spots” becomes a reliability issue since the overheating reduces the life time of the device. Briefly stated, a good impedance match ensures the signal power is transmitted to the RF load instead of being dissipated as heat.
Providing an impedance match at a single frequency is conventional and not difficult. On the other hand, achieving a good impedance match over a wide frequency band is usually challenging. And this is exactly what is needed. The modern communication system continues to evolve into one that demands ever increasing bandwidths. Thus, the need for an impedance matching solution for wide band applications is more critical than ever. In particular, the optimum matching impedance of a RF power transistor is fairly low in power amplifier designs. For example, in the LTE bands of 700 to 2700 MHz a matching network configured to transform a low RF transistor impedance to the system impedance is usually a design challenge.
In one approach, wideband matching using a lumped element network has been considered. However, tolerance variations and parasitic effects of the lumped element components make unfeasible for high frequency designs.
At high frequencies, distributed transmission line matching circuits are generally the preferred approach. In this type of solution, the common technique is to employ multiple sections of quarter wavelength transmission line or stepped impedance transformers that have certain impedance profiles along the line. Referring to FIG. 1, for example, a schematic diagram of a conventional four-section Chebyshev impedance transformer is shown. The Chebyshev impedance transformer uses four sections of quarter wavelength transmission lines to convert a high impedance of Zs to low impedance of Zs/4. The impedances for each stage are shown in the FIG. 1. Without loss of generality, the center frequency is chosen as 1.75 GHz and the high impedance Zs is chosen as 50 ohm The total size of the Chebyshev transformer is one wavelength at the center frequency. Unfortunately, the improvement in bandwidth of these transformers is outweighed by the substantial physical size of these circuits, thus generally leading to higher loss. Stated differently, bulky solutions are not compatible with the current miniaturization trend in the wireless communications industry.
In yet another approach, a matching arrangement that includes a plurality of coaxial transmission lines has been considered. In this arrangement, each transmission line is wound around a ferrite toroid for a predetermined number of turns, or inserted into ferrite sleeves to achieve wideband impedance match. There are, however, drawbacks associated with this approach. Like the distributed transmission line approach, the use of coaxial transmission lines is a bulky solution that is not favored for the aforementioned reasons. Moreover, the ferrites exhibit a limited operating frequency band due to increased losses at high frequencies.
What is needed, therefore, is a relatively compact wideband impedance transformer that substantially overcomes the drawbacks articulated above. A compact wideband impedance transformer is needed that does not, for example, employ ferrites or other bulky features.